1. Field of the Invention
The present invention relates to a digital broadcasting system which permits the users to receive a variety of services not only through a limited number of physical channels through which program data are actually transmitted but also through one or more virtual channels.
2. Description of the Prior Art
Virtual channels have been proposed in the ATSC (Advanced Television Systems Committee) standard published in 1996. According to the standard, virtual channels offer the user a constant view of services available on any particular transport stream (TS) compliant with the standard. The virtual channels are specified in the virtual channel table (VCT) whose records are transmitted with service data.
If a receiver used in a broadcasting system compliant-with the standard is turned on after such a long interval that the stored VCT is no longer effective, the normal operation of the receiver will not be guaranteed. Also, in the broadcasting system, any channel can be changed to another physical channel only in accordance with the timing defined in a VCT. However, the broadcaster may desire to change one or more channels to any other physical channel or a specific physical channel during ongoing programs of the one or more channels for some reason, e.g., alternative broadcasting in case of a trouble in any of program source systems in the broadcasting center or announcing the highlight of ongoing program of the specific physical channel.
It is therefore an object of the invention to provide a digital broadcasting system which utilizes virtual channels interchangeable at any time by frequently inserting channel shift information into each of the transport streams broadcast from a broadcasting center.
It is another object of the invention to provide some forms of digital broadcasting systems using virtual channels.
A digital broadcasting system according to the principals of the invention can cause one of a number of physical channels selected by a broadcaster to be received as one of the channels available to users. For each of the programs of each of the channels available to said users, a program information record comprising PSI (program specific information) is stored in a data base. The broadcaster is permitted to include, in the PSI, channel mapping information (CMI) whose mapping destination is a channel (a program) to be received instead of a program for which said program information record is intended or a channel through which the program to be received is transmitted. In other words, a CMI includes either a program or a channel ID as mapping destination. The program information records is inserted in broadcast transport streams on schedule (i.e., at predetermined times for respective programs) and, if necessary, immediately after the broadcaster has included the channel mapping information in the PSI data.
Some strategies of selecting a series of programs for a virtual channel are proposed.